1. Field of the Invention
The invention relates to fabrication of optical fiber from silica preforms, particular preforms comprising overcladding and/or substrate tubes formed from sol-gel processes.
2. Discussion of the Related Art
Glass optical fiber is produced from a glass preform. As discussed in F. DiMarcello et al. xe2x80x9cFiber Drawing and Strength Properties,xe2x80x9d Optical Fiber Communications, Vol. 1, Academic Press, Inc., 1995, at 179-248, the disclosure of which is hereby incorporated by reference, the preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down legion, the optical fiber is drawn.
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816, the disclosures of which are hereby incorporated by reference. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform, as discussed in U.S. Pat. No. 4,775,401, the disclosure of which is hereby incorporated by reference.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass does not have to meet the optical performance specifications to which the core and the inner cladding must conform. For this reason, efforts to both ease and speed manufacture of fiber preforms focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process.
U.S. Pat. No. 5,240,488 (the ""488 patent), the disclosure of which is hereby incorporated by reference, discloses a sol-gel casting process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In particular, the ""488 patent describes use of polymer additives, e.g., binders and lubricants to improve the physical properties of the gelled bodies. According to the ""488 patent,.a silica dispersion is provided, and stabilized by addition of a base such as tetramethylammonium hydroxide. Polymer additives are mixed in, and the mixture is allowed to age. A gelling agent is added to induce gellation, and, typically, once the gelling agent is added, but before gellation occurs, the mixture is pumped into a tubular mold containing a central mandrel, in which the gel is aged for 1 to 24 hours. The mandrel is removed, and the gelled body is then extracted from the mold, typically by launching the body from the mold in water to prevent breakage. The body is then dried, heated to remove volatile organic materials, dehydroxylated (typically by a chlorine treatment), and then sintered to form the finished overcladding.
As discussed in U.S. Pat. No. 5,344,475 (the ""475 patent), the disclosure of which is hereby incorporated by reference, the presence of refractory oxide particles, e.g., zirconia, chromia, in a silica overcladding tube reduces the strength of the resultant fiber because fracture tends to begin at such flaws. The presence of such oxide particles is typically unavoidable in a sol-gel process, however, due to contamination introduced both in the initial silica dispersion and by the mixing and molding equipment. Techniques were therefore developed for reducing to acceptable levels the concentration of such particles in sol-gel overcladding tubes. The ""475 patent describes the use of centrifugation prior to gellation of the sol to remove such particles, based on the difference in size and/or density between silica particles and the refractory oxide particles. U.S. Pat. No. 5,356,447 (the ""447 patent), the disclosure of which is hereby incorporated by reference, describes a further process for reducing the amount of refractory oxide particles in a gelled tube. The process relies on the treatment of the gelled tube by a chlorine-containing gas, e.g., thionyl chloride (SOCl2). The process of the ""447 patent is useful in preparing overcladding tubes for fiber preforms, but improvements and/or alternatives for removing refractory oxide particles from sol-gel bodies are being sought.
Refractory oxide particles are removed and/or reduced in size, in a silica body, e.g., a sol-gel derived overcladding or substrate tube, by treatment with a gaseous mixture containing one or more non-oxygenated sulfur halides. The treatment is also useful for dehydroxylating such a body. (A non-oxygenated sulfur halide is a compound containing sulfur and one or more halides, but not containing oxygen, e.g., sulfur fluorides, sulfur chlorides, sulfur iodides, sulfur bromides, as well as combinations of halides, e.g., sulfur chloro-fluorides. Combinations of different halide compounds are possible, e.g., mixtures of sulfur chlorides and sulfur fluorides. Treatment with a gaseous mixture containing a non-oxygenated sulfur halide indicates that the majority of the non-oxygenated sulfur halide or halides are formed in a reactor or furnace or introduced into the reactor or furnace by a manner other than decomposition inside the reactor or furnace of an oxygenated sulfur halide. Oxygenated sulfur chlorides include compounds containing sulfur, oxygen, and one or more halides, such as thionyl chloridexe2x80x94SOCl2.) Due to the resultant reduction in the concentration and/or size of refractory metal oxide particles in the overcladding tube, optical fiber drawn from a preform fabricated from the treated silica body exhibits desirable strength.
Advantageously, the sulfur halide contains one or more sulfur chlorides or sulfur chloro-fluorides. Sulfur chloride includes compounds of the general formula SxCly, e.g., sulfur monochloride (S2Cl2), sulfur dichloride (SCl2), the chlorosulfanes (S2Cl2), sulfur tetrachloride (SCl4), and compounds containing the radical SCl or compounds that generate the ion SCl3+. (See, e.g., Gmelin Handbuch der Anorganischen Chem, S Eng. 2, Springer-Verlag, 1978.) Sulfur chloro-fluorides include dimerized sulfur dihalide (S2ClF3), disulfur chloride fluoride (S2ClF), sulfur chloride pentafluoride (SF5Cl). A sulfur chloride treatment provides desired improvements compared to treatment by thionyl chloride. For example, sulfur monochloride exhibits a zirconia removal rate from a silica gel body about three times faster than thionyl chloride at 600xc2x0 C. In addition, whereas processes using thionyl chloride sometimes require an additional chlorine-treatment step to remove chromia at a desired rate, a sulfur chloride treatment is generally capable of removing chromia at an acceptable rate without such chlorine treatment. Moreover, when a sulfur chloride treatment is used instead of a thionyl chloride treatment, the chlorine remaining in the silica body is more easily and quickly removed by subsequent oxygen treatment. Also, sulfur monochloride is a relatively inexpensive compound capable of being purchased at high purity levels, and the compound lends itself to on-demand formation in a production plant. In fact, sulfur chlorides, as well as other sulfur halides, are capable of being formed by an in situ process of incorporating sulfur into the sol-gel body and then introducing the gaseous halides into the furnace containing the body.
Treatment by sulfur halides thus constitutes an improved technique useful in forming silica bodies for fiber preforms, particularly sol-gel derived overcladding or substrate tubes, the silica bodies contributing to improvements in the overall fiber fabrication process.